1. Field of the Invention
The invention relates to a process for deaggregating highly disperse silicic acid in which average particle sizes of less than 100 nm, preferably less than 50 nm, are achieved by exposure to extremely high mechanical stress. The deaggregated silicic acid may be employed as a filler in silicon rubber mixtures.
2. Description of the Background
Diverse possible uses are known for highly disperse silicic acids prepared by pyrogenic or wet chemical processes. A main field of use is to employ these products as so-called reinforcing or active fillers in silicone rubber mixtures. These silicone rubber mixtures essentially comprise polydimethysiloxanes which have a sufficient number of reactive groups in order to convert the silicone rubber mixture into a rubber-elastic state with the aid of crosslinking substances and, if appropriate, catalysts, and the pyrogenic silicic acids mentioned, which usually have BET surface areas of at least 50 m.sup.2 /g. Depending on the nature of the crosslinking, a distinction is made, for example, between 1- or 2-component systems which crosslink at room temperature (RTV1, RTV2) and systems which crosslink at high temperature (HTV).
Regardless of the nature of the crosslinking, the properties of the products of all the systems are also determined decisively by the silicic acid employed. In addition to the size of the specific surface area of the silicic acid, above all the surface treatment of the silicic acid and the distribution of the silicic acid in the polysiloxane matrix are of import for the product characteristic values, both of the non-crosslinked silicone rubber mixture and of the finished elastomer.
The size of the silicic acid surface can be varied during the preparation of the mixture only within narrow limits. In contrast, the surface treatment of the silicic acid can be carried out to a varying intensity both during and after its incorporation into the polysiloxane. During this process, Si--OH groups bonded on the silicic acid, which otherwise have the effect of stiffening the silicone rubber mixture, are blocked chemically. The so-called in Situ hydrophobization is carried out with a reactive organosilicon compound, usually with hexamethyldisilazane.
In addition, the distribution of the silicic acid can be greatly influenced. Highly disperse silicic acids have a specific structure (Degussa: Schriftenreihe Pigmented NR. 11 [Pigment Publication Series No. 11], 1993, page 26). The primary particles having a size of between 7 nm and 40 nm, and are agglomerated into aggregates, which in turn join together to form even larger structures, agglomerates.
To achieve product properties which represent a combination of the best possible flow properties of the non-crosslinked silicone rubber and at the same time high mechanical strength of the crosslinked silicone rubber, such as is necessary, for example, in two-component systems employed as impression materials, the structure of the silicic acid must be broken down as far as possible. Small particle sizes, as far as possible primary particles, are aimed for in the rubber. As is known, larger structures are broken down by introducing shear energy, by subjecting the mixture of polysiloxane and silicic acid to greater or lesser mechanical stress.
German Patent Application DE 25 35 334 describes a process for homogeneous distribution of highly disperse fillers into polysiloxanes. In this process, the filler is hydrophobized in situ during mixing into the polymer, and the mixture is then subjected to a mechanical treatment. Although the silicone rubber mixtures prepared by this process are storage-stable, they do not have the properties required for impression materials because of inadequate breakdown of the agglomerated silicic acid particles.
A Drais mixer with the smallest possible gap width is described in DE 195 07 878 as a special mixing unit for effective introduction of shear energy. The average particle size which can be achieved is 100 to 200 nm. In this process also, the filler is hydrophobized in situ.
An extrudable material comprising polymer and filler is described in DE 44 42 871. For this material, 33 to about 65% by weight of filler is combined with a polymer under conditions of a relatively high shear action, and the components are then mixed, while maintaining the high shear forces. Particle sizes of 100 to 200 nm are obtained.
Another possibility of achieving small silicic particles is described in EP 622 334. In this case, pyrogenic silicic acid is obtained immediately with a particle size between 40 and 200 nm in a special process by combustion of a chlorosilane in a mixture of oxygen, hydrogen and, if appropriate, nitrogen. The silicic acid obtained is then employed for the preparation of silicone rubber mixtures without a further change in its structure, and hydrophobized in situ (EP 622 421).
No processes are described for deaggregation of aggregated and agglomerated highly disperse silicic acid in silicone rubber mixtures in which the silicic acid in the mixture finally has average particle sizes of less than 100 nm, down to the primary grain.